The overall goal of this project is to identify molecular mechanisms for self-renewal and survival of leukemic initiating (stem) cells that may be targets for curative therapy of human Philadelphia chromosome-positive (Ph+) leukemias. Ph+ leukemias induced by the BCR-ABL oncogene, including chronic myeloid leukemia (CML) and the B-cell acute lymphoblastic leukemia (B-ALL), are among the most common myeloproliferative malignancies. Allogeneic bone marrow transplantation, the only established curative therapy for CML, is not feasible for most patients. The BCR-ABL tyrosine kinase inhibitors including imatinib mesylate (Gleevec) are highly effective in treating chronic phase CML patients, but not more advanced CML blast crisis and Ph+ B-ALL patients. Moreover, clinical resistance to imatinib develops and imatinib does not completely eradicate leukemic stem cells, suggesting that use of a BCR-ABL kinase inhibitor such as imatinib as a single agent will not prevent eventual disease progression or cure CML. New therapeutic strategies are needed, especially for older CML patients. Our overall hypothesis is that self-renewal and survival of these leukemic stem cells are mediated by different molecular pathways than those that stimulate leukemic cell proliferation, and targeting leukemic stem cells is essential to curative therapy of Ph+ leukemias. This project uses the efficient, accurate mouse models for Ph+ leukemias to identify and characterize leukemic stem cells by focusing on three specific aims: 1) To determine which hematopoietic lineages of BCR-ABL-transduced bone marrow cells transfer Ph+ leukemias, affect leukemic stem cell function, and are sensitive to BCR-ABL kinase inhibitors. Leukemic mice will be treated with a BCR-ABL kinase inhibitor imatinib or BMS-354825, followed by the analysis of leukemic stem cells using multicolor FACS to identify and sort different cell lineages for adoptive transfer of the diseases and for examination of survival/self-renewal of the stem cells. 2) To determine the hematopoietic lineages in which 2-catenin is activated by BCR-ABL, to test whether Src kinases are involved in 2-catenin activation in these lineages and have a positive effect on survival of these cells, and to establish the role of 2-catenin in BCR-ABL leukemogenesis. Src kinase-deficient mice and Src kinase inhibitors, and 2-catenin conditional knockout mouse will be used to address the role of 2-catenin in BCR-ABL leukemogenesis. 3) To determine the inhibitory mechanism of a novel anti-stem cell agent BMS-214662 on survival of BCR-ABL-expressing HSCs and to test whether this compound in combination with a BCR-ABL kinase inhibitor provides a curative therapy for Ph+ leukemias. Leukemic mice will be treated with BMS-214662 alone or together with a dual BCR- ABL/Src kinase inhibitor BMS-254825, followed by the analysis of survival/self-renewal of leukemic stem cells in mice. This study will help to understand the biology of leukemia stem cells and develop novel anti-stem cell strategies for Ph+ leukemias.